The present invention relates to a thermal transfer sheet, and more particularly to an integral thermal transfer medium comprising a thermal transfer film having a hot-melt ink layer (an ink-providing material) and an image receiving sheet (a material on which the ink layer is to be transferred to form an image) temporarily bonded to the thermal transfer film.
A thermal transfer film (an ink film) comprising a substrate film and a hot-melt ink layer provided on one surface of the substrate film has hitherto been used as a thermal transfer recording medium for thermal printers, facsimile machines, etc. Printing on paper using the thermal transfer film is effected by a method which comprises feeding a thermal transfer film from a roll on which the thermal transfer film has been wound, separately feeding an image receiving sheet in a continuous or sheet form, putting both materials on top of the other on a platen, and applying heat in this state with a thermal head from the back surface of the substrate film to form a desired image.
These thermal transfer media, however, cannot be applied to, for example, a facsimile printer using the conventional thermosensitive coloring paper because the conventional facsimile printer effects printing by taking advantage of thermal coloring of recording paper per se and is not provided with a mechanism for separately carrying a thermal transfer film (an ink film) and an image receiving sheet.
In order to solve the above-described problem, a method has been devised wherein the thermal transfer film is temporarily adhered in advance to the image receiving sheet and the laminate is rolled and applied to the facsimile printer.
This co-rolled thermal transfer medium comprising a thermal transfer film and an image receiving sheet in an integral form has an adhesive layer that serves to adhere the thermal transfer film to the image receiving sheet and enables the thermal transfer film and the image receiving sheet to be peeled from each other after the completion of the thermal transfer. Since a material having a low softening point, such as a sticking agent, is mainly used in the conventional ink, the initial adhered state changes due to the occurrence of unfavorable phenomena, such as creeping, softening and melting with time or during storage at a high temperature. Such changes cause problems of an abnormal transfer of the hot-melt ink layer to the image receiving sheet when the transfer film is peeled from the image receiving sheet. Further, in the conventional co-rolled thermal transfer medium, since the thermal transfer film and the image receiving sheet are adhered to each other in a face-to-face manner, when use is made of a material having a high adhesive force, such as an adhesive resin, the above-described problem becomes more significant and problems particularly associated with the storage and peeling occur.